1. Field of the Invention
The present invention pertains to electrostatic reproduction of documents and, more particularly, to method and apparatus incorporating charge transfer to electrostatically reproduce copies from opaque or transparent originals.
2. Discussion of the Prior Art
There have been basically two techniques commercially utilized for electrophotographic reproduction. In the first method, a latent charge image is formed on a photoconductive surface on the copy sheet itself and the latent image is developed in situ by means of a charged liquid or powder developer or toner. This method of electrophotographic reproduction is relatively expensive due to the fact that each copy sheet must carry a photoconductive layer. In the second method, a latent charge image is formed on an intermediated photoconductive surface, for example a selenium coated drum, and the latent image is developed by applying an electrostatic powder thereto, the developed powder image then being transferred to a copy sheet and fixed. The step of transferring the developed image to the copy sheet, which in principle is less expensive due to the decreased expense of the copy sheet, nonetheless has the disadvantage that the latent image must be developed prior to transfer. Accordingly, the transfer of the developed powder image, of necessity, requires precise, delicate apparatus, careful maintainance and constant cleaning of the photoconductive surface.
Due to the above-mentioned disadvantage in the transfer of powder images, attempts have been made to improve the second method by transferring the latent image formed by the electrostatic charges on the photoconductive surface rather than a developed image. As exemplified by French Pat. No. 1,110,794, one attempt to transfer the electrostatic charge image has included the placing of a dielectric surface above a conductive electrode and disposing a photoconductive layer associated with a transparent conductive support against the dielectric surface. While the photoconductive layer is exposed to a light image, an electric field is applied between the electrode supporting the dielectric surface and the conductive support for the photoconductive layer. This method does not represent true charge transfer in that the photoconductive layer is not previously charged. That is, this method operates by the creation of a charge image directly on the dielectric surface and, thus, constitutes more accurately a process of dielectric writing. In a variation of this method, a transparent electrode can be utilized as a support for the dielectric surface with the latent image being formed by means of the transparent electrode. A major disadvantage of this method of overcoming the problem of charge transfer is that the method is essentially static; that is, the method must be performed while the components thereof are stationary.
A method similar to that described above but which can be applied dynamically is exemplified by French Pat. No. 1,488,489 and uses photoconductors with persisting conductivity by forming zones of differential conductivity through the transparent support for the photoconductive layer. This method is also essentially a method of dielectric writing rather than a transfer process in that the photoconductive layer is not previously charged.
Another method attempting to overcome the problem of transfer of powder images, as exemplified by French Pat. No. 1,413,952, applies a charge image previously formed on a photoconductive surface to a dielectric surface under the effect of high pressure. The charge transfer, which is efficient, is performed under the effect of the field created only by the charges deposited on the photoconductive surface without the necessity of an external electric field and the charge transfer occurs when the resultant of the charge field exceeds the value of an ionizing field. This method, while being dynamic in nature, requires that the photoconductive and dielectric surfaces be intimately contacting one another under high pressure; however, even under the best of conditions, only part of the charge is transferred and the results of the method are not acceptable. Similarly, it has been proposed, for example as shown in French Pat. No. 2,024,150, to use certain photoconductors that accept very high charge levels in an attempt to permit dynamic transfer of latent charge images onto a dielectric surface without requiring excessively high pressure and also without necessitating an external field; however, this method has the disadvantage of requiring a high potential which, in turn, requires expensive apparatus and undue limitations on the user.
Other methods of transferring latent charge images utilize the effect of an external field to transfer the charge onto a previously charged insulating surface from a photoconductor surface, as exemplified by French Pat. No. 1,105,940 wherein a latent image previously formed on a photoconductive surface is transferred to an insulating surface which has previously been uniformly charged. This method utilizes an appropriate external field or an excessively high charge applied to the image to be transferred in order to avoid the requirement of high mechanical pressure; this method, which operates by destruction of charges previously deposited on the insulating surface and not by true charge transfer, yields only mediocre results due to the fact that the insulating surface often contains persistent unwanted charges causing an undesired background in the reproduced copy and, further, this method is, of necessity, of a static nature. Because of the presence of an external field sufficiently strong to destroy all previously deposited charges on the insulating surface, there are parasitic transfers which disturb the light zones in the reproduced image. The above-mentioned disadvantages are also apparent when this method is improved, as suggested by British Pat. No. 855,727, by using a pulsed current to form the electric field between the electrodes.
An attempt to utilize the immediately above described method dynamically, for example as disclosed in French Pat. No. 1,531,688, has utilized an electric field formed of a continuous component and a high frequency component to transfer a latent image formed on the photoconductive surface of a drum to an insulating sheet that has previously been uniformly charged and is supported by a second drum, the two drums rotating tangentially at the same peripheral speed in immediate proximity to each other. This method creates an ionized zone between the two drums, such ionized zone being persistent by the high frequency current and not being instantaneous. An improvement of this method contemplates extending the dimensions of the ionized zone by utilizing a conductive grid; however, such improvement does not eliminate the background that persists in the light zone of reproduction or the familiar parasitic discharges.